How to use generics in Haskell -

i'm trying wrap head around generics in haskell. issue created concatenate function takes list of strings , returns giant string.

concatenate:: [string] -> string concatenate xs = foldl (\acc x -> acc ++ x) [] xs 

but i'd instead of list of strings, use list of anything. can strings, can ints.

concatenate ["phil", "is"]  

generates "philis" while

concatenate [[1,2],[3,4]] 

generates [1,2,3,4].

i've found

concatenate:: [a] -> concatenate xs = foldl (\acc x -> acc ++ x) [] xs 

does not work , i'm not sure why. isn't way haskell works, whatever type is, output well? or issue second half not allowing work function types?

keep in mind string alias [char].

what intended this:

concatenate:: [[a]] -> [a]